Silicon carbide reinforced reaction bonded silicon carbide composite

ABSTRACT

The invention includes a process for producing a reaction bonded silicon carbide composite reinforced with coated silicon carbide fibers which is suitable for high temperature applications. The process includes the steps of coating SiC fibers with AlN, BN or TiB 2  ; treating the coated fibers with a mixture of SiC powder, water and a surfactant; preparing a slurry comprising SiC powder and water; infiltrating the coated fibers with the slurry to form a cast; drying the cast to form a green body; and reaction bonding the green body to form a dense SiC fiber reinforced reaction bonded matrix composite. 
     The invention further includes a SiC fiber reinforced SiC composite comprising a reaction bonded SiC matrix, a SiC fiber reinforcement possessing thermal stability at high temperatures and an interface coating on the fibers having chemical and mechanical compatibility with the SiC matrix and with the SiC fibers.

This is a divisional application of U.S. application Ser. No.08/698,740, filed Aug. 16, 1996 now U.S. Pat. No. 5,817,432; which is adivisional of U.S. Ser. No. 08/447,148, filed May 22, 1995 now U.S. Pat.No. 5,643,514; which is a continuation of U.S. application Ser. No.08/150,649, filed Nov. 4, 1993, now U.S. Pat. No. 5,484,655; which is adivision of U.S. application Ser. No. 07/852,589, filed Mar. 17, 1992,now U.S. Pat. No. 5,296,311.

FIELD OF INVENTION

This invention relates to dense matrix composites suitable for hightemperature applications. More particularly, this invention relates toSiC fiber reinforced reaction bonded SiC composites wherein the SiCfibers are coated with a ceramic material.

BACKGROUND OF THE INVENTION

Reinforced ceramic matrix composites are well suited for structuralapplications because of their potential toughness, thermal resistance,high temperature strength and chemical stability. These composites canbe produced by the addition of whiskers, fibers or platelets to aceramic matrix. The non-brittle nature of these composites provides themuch needed reliability that is otherwise lacking in monolithicceramics.

Fabrication of ceramic matrix composites reinforced with sinteredcontinuous fibers is more difficult than fabrication of dense monolithicceramics. Conventional sintering of a green ceramic matrix reinforcedwith sintered fibers is not possible if the green ceramic matrix hasrigid inclusions. Densification can, however, be achieved by chemicalvapor infiltration (CVI) or reaction bonding. Reaction bonding is thepreferred method because it is less time consuming and more oftenproduces a fully dense body than the CVI process. For high temperatureapplications, full densification is necessary to prevent rapid oxidationdegradation of the reinforcements or reinforcement coating.

Densification by reaction bonding, described in U.S. Pat. No. 3,205,043to Taylor, involves infiltrating molten silicon through the pores of agreen body containing SiC and carbon. The silicon reacts with the carbonto form SiC, which then bonds the SiC grains together. In the absence ofcarbon, the infiltrated molten silicon solidifies upon cooling, therebyfilling the pores of the SiC bonded SiC body. This process is known assiliconization. The resulting fully dense end product contains SiC andresidual free silicon. Since reaction bonding does not involve shrinkageof the green body as does conventional sintering, the final denseproduct is a near net shape.

Fracture resistance of ceramic matrix composites is achieved throughcrack deflection, load transfer, and fiber pull-out. Fiber pull-out,which is well established as central to the toughness of ceramic fibercomposites, is achieved by having little or no chemical bonding betweenthe fibers and matrix. The fibers must be able to readily debond andslide along the matrix for increased fracture toughness of thecomposite.

It is known that many fiber-matrix combinations undergo extensivechemical reaction or interdiffusion between the fiber and matrixmaterials, each of which is likely chosen for the contribution ofspecific mechanical and/or physical properties to the resultingcomposite. Such reaction or interdiffusion can lead to seriousdegradation in strength, toughness, temperature stability and oxidationresistance. The fiber-matrix interface is therefore very important topreventing or minimizing chemical reactions and interdiffusion.

Surface modification of the fibers is an effective means to control thefiber-matrix interface. This can be accomplished by coating the fiberswith a suitable ceramic to inhibit the fibers from reacting or bondingwith the matrix. The ceramic coating allows the fiber to pull out fromthe matrix and slide along the matrix, thus increasing the fracturetoughness of the composite.

Coated silicon carbide fibers and whiskers are known reinforcements forcomposite materials. U.S. Pat. No. 4,929,472 to Sugihara et al.discloses SiC whiskers having a surface coated with a thin, 7-100 Å,carbonaceous layer and SiC whiskers coated with a Si₃ N₄ layer which is15-200 Å thick. These surface coated whiskers are used as a reinforcingmaterial for ceramics such as SiC, TiC, Si₃ N₄, or Al₂ O₃.

U.S. Pat. No. 4,781,993 to Bhatt discloses a SiC fiber reinforcedreaction bonded Si₃ N₄ matrix wherein the SiC fibers are coated with anamorphous carbon layer and an overlayer having a high silicon/carbonratio covering the amorphous layer.

U.S. Pat. No. 4,642,271 to Rice discloses BN coated ceramic fibersembedded in a ceramic matrix. The fibers may be composed of SiC, Al₂ O₃or graphite, while the matrix may be composed of SiO₂, SiC, ZrO₂, ZrO₂--TiO₂, cordierite, mullite, or coated carbon matrices.

U.S. Pat. No. 4,944,904 to Singh et al. discloses a composite containingboron nitride coated fibrous material. Carbon or SiC fibers are coatedwith BN and a silicon-wettable material and then admixed with aninfiltration-promoting material. This mixture is then formed into apreform which is then infiltrated with a molten solution of boron andsilicon to produce the composite.

Teusel et al. in "Aluminum Nitride Coatings on Silicon Carbide Fibres,Prepared by Pyrolysis of a Polymeric Precursor", J. Mat. Sci., 25 (1990)3531-3534, discloses a method of coating SiC fibers. Nicalon (SiC)fibers, Nippon Carbon Co. Ltd., were thermally pretreated in nitrogenand dip coated with a solution of metallic aluminum in an organicelectrolyte. The fibers were then calcined at 900° C. under anhydrousammonia. The authors found that a thin coating, about 0.5 microns,produced a smoother and more uniform surface than thicker coatings. Theperformance of these AlN coated SiC fibers in a SiC matrix was notdiscussed in the Teusel et al. article.

A specific problem encountered with SiC reinforced SiC composites isthat the SiC fibers or coatings on the SiC fibers may react with thematrix during formation of the composite, resulting in a strongfiber-matrix bond. This strong interfacial bond leads to decreasedfracture toughness. It is an object of the invention, therefore, toprovide a process for incorporating SiC fibers into a SiC matrix whilecontrolling the fiber-matrix interface to achieve high fracturetoughness. It is also an object of the invention to provide a processfor producing a SiC composite possessing high temperature strength.

SUMMARY OF THE INVENTION

The present invention has resulted from the discovery that a reactionbonded silicon carbide composite reinforced with coated silicon carbidefibers can produce a dense ceramic composite suitable for hightemperature applications. AlN, BN and TiB₂ coatings were found to limitboth mechanical and chemical bonding with the matrix to improve thestrength and toughness of the composite material.

The present process for producing SiC fiber reinforced SiC compositesincludes the steps of coating SiC fibers with a composition selectedfrom the group consisting of AlN, BN and TiB₂ ; treating the surface ofthe coated fibers with a mixture of SiC powder, water and a non-ionicsurfactant; preparing a slurry comprising SiC powder and water; vacuuminfiltrating the coated fibers with the slurry to form a cast; dryingthe cast to form a green body; and reaction bonding the green body toform a dense SiC fiber reinforced reaction bonded matrix composite.

The SiC reinforced SiC composite of the present invention includes areaction bonded SiC matrix, a SiC fiber reinforcement possessing thermalstability at temperatures of at least 1420° C., preferably 1500° C. andan interface coating on the fibers having chemical and mechanicalcompatibility with the SiC matrix and with the SiC fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are 1000 times and 1500 times magnified microscopicphotographs showing the fracture surfaces of AlN coated SiC fibers.

FIGS. 3 and 4 are micrographs of the reinforced composite manufacturedin accordance with the process of the present invention.

FIG. 5 is a 200 times magnified microscopic photograph showing fiberpull out at the fracture surface of a reaction bonded SiC matrixcomposite incorporating AlN coated SiC fibers.

FIG. 6 is a 200 times magnified microscopic photograph showing fiberpull out at the fracture surface of a reaction bonded SiC matrixcomposite incorporating BN coated SiC fibers.

FIG. 7 is a photograph showing the stable crack growth of a reactionbonded SiC composite containing BN coated SiC fibers.

FIG. 8 is a graph showing the load-deflection curves for both a BNcoated SiC fiber reinforced reaction bonded SiC composite according tothe present invention and a monolithic reaction bonded SiC.

FIG. 9 is a 250 times magnified microscopic photograph of the fracturesurface of uncoated SiC fiber reinforced reaction bonded SiC matrixcomposite showing no fiber pullout.

DETAILED DESCRIPTION OF THE INVENTION

The subject invention relates to a process for producing SiC fiberreinforced reaction bonded SiC matrix composites wherein the fibers arecoated with a non-oxide ceramic material and the article thus produced.The matrix material provided in the present invention is reaction bondedSiC which possesses net shape processing capability and ease offabrication.

The SiC fibers employed in the present invention are sinteredpolycrystalline SiC fibers from The Carborundum Company, Niagara Falls,N.Y. However, other SiC fibers, such as those produced by chemical vapordeposition or other processes could be used if they are suitable for usein the reaction bonding process, especially those possessing thermalstability at 1420° C. or higher. Most of the existing commercialprecursor-derived SiC fibers such as Nicalon (Nippon Carbon Company) andTyranno (UBE Industries, Japan) are not suitable for this applicationbecause they lack the thermal stability necessary for use in reactionbonded SiC composite fabrication.

To achieve the desired composite properties, namely, high temperaturestrength and fracture toughness, it is necessary that there is asuitable interface coating between the matrix and the fiber. To providenon-catastrophic failure of the composite, the fracture energy must bedissipated by the fiber pulling out from the matrix and sliding alongthe matrix. The frictional sliding expends the energy, thereby providingincreased fracture toughness. If uncoated SiC fibers are incorporatedinto a matrix of reaction bonded SiC, fiber pull out is not achievedbecause of the bonding of the SiC fiber with the SiC matrix. Thus, anycrack that occurs in the surface of the composite will propagate throughthe matrix and continue through, i.e., transverse to, the fiberresulting in the typical brittle fracture behavior of conventionalmonolithic ceramics.

The preferred non-oxide ceramic coatings for the SiC fiberreinforcements are AlN, BN and TiB₂ and combinations thereof. A furtherpreferred fiber coating is AlN because AlN exhibited the most desirablefiber pull out behavior with the reaction bonded SiC matrix.

The non-oxide ceramic can be coated onto SiC fibers by several methods,including, (1) chemical vapor deposition, (2) evaporation of aluminum,followed by chemical conversion with NH₃, and (3) deposition of an Al₂O₃ coating through sol-gel, followed by chemical conversion with NH₃.Chemical vapor deposition is the preferred method because it is mostconvenient and thus far has produced the most uniform coatings.

An AlN coating thickness of between about 1-15 microns on the SiC fibersis desired. A BN coating thickness of between about 0.1 to 10 microns isdesired. The preferred thickness of BN is between about 0.5 to 2microns. We have found that thin AlN coatings, less than 1 micron, wereinadequate because the AlN actually dissolved in and reacted with themolten silicon during the infiltration process. After incorporatingthinly coated fibers into the reaction bonded SiC matrix, no AlN coatingnor fiber pull out could be detected. However, when sintered fiberscoated with a thicker layer of AlN were incorporated into the reactionbonded SiC matrix and fractured in a four-point bending test, fiber pullout could be observed. This is demonstrated in FIGS. 5 and 6. Althoughsome AlN may react or dissolve in molten silicon during reactionbonding, fiber pull out will still occur as long as there remains someunreacted AlN coating on the SiC fibers. The AlN coating remaining onthe SiC fibers after reaction bonding is between 0.1 to 15 micronsthick.

A green body of coated SiC fiber reinforced SiC composite is preferablyproduced by a slurry filtration process. In this process, the slurry isprepared by ball milling SiC powder (for example, submicron SiC powdermarketed by Arendal Smelteverk A.S., Norway) in water. To ensure gooddispersion of the powder, the pH of the slurry is adjusted to between 8and 10 by adding ammonium hydroxide to the slurry. A small amount, about0.5 wt %, of sodium silicate may be used as a binder. Other binders thatcan be used include PVA, sucrose syrup, phenolic, acrylic latex andother water soluble binders. The solid content of the slurry ispreferably between 20 and 80 wt %. The slurry is then poured into amold.

An appropriate amount of sintered SiC fiber in the form a bundle isdipped into a mixture of SiC powder and water (solid content of 10-50wt. %) containing about 2% or less of a non-ionic wetting agent, such as2 wt % Triton x-100 surfactant, comprisingiso-octylphenoxypolyethoxyethanol. The surfactant treated fiber bundleis then laid in the SiC slurry and infiltrated and dewatered. Theresulting cast is allowed to fully dry to form the green body. The greenbody is then completely densified by conventionalsiliconization/reaction bonding. The temperature range forsiliconization/reaction bonding is between 1420° C. (the melting pointof silicon) and 2400° C., and preferably between 1500 and 1600° C. Theprocess is preferably carried out under vacuum to prevent oxidation, butcan be carried out in atmospheric pressure. Complete densification isachieved at temperatures as low as 1500° C. in 1/2 to 1 hour undervacuum for small test samples. Densification time and temperature dependon the size of the article and on the carbon content in the slurry.

Carbon as particulate carbon, colloidal carbon, or carbon-yieldingresins may also be added to the SiC slurry. However, it is importantthat the rheology and chemistry of the slurry is not severely altered.The added carbon can help achieve good wicking of molten silicon duringthe reaction bonding process and help minimize residual silicon in thedense body by reacting with the Si to form SiC.

Composites with fiber volume fraction as high as 0.44 were alsoproduced. Composites produced without the addition of wetting agent tothe slurry had regions where veins of silicon were present. However,when the wetting agent is used, these silicon veins were absent and auniform microstructure was obtained.

A fracture toughness of about 13 MPa m^(1/2) was calculated from thefiber pull out lengths for a reaction bonded SiC matrix composite havingabout 40 vol. % of AlN coated fibers. Similar fiber pull out for BNcoated SiC fibers in a reaction bonded SiC matrix has been observed, asis demonstrated in FIG. 6. Because AlN has a higher oxidation resistancethan BN, it is expected that the AlN coated SiC fiber reinforcedcomposite will have a higher oxidation resistance than the BN coated SiCfiber reinforced composite.

The SiC fiber reinforced reaction bonded composite of the presentinvention possesses thermal stability up to 1420° C.

FIGS. 3 and 4 are micrographs of the reinforced composite manufacturedin accordance with the process of the present invention.

The examples which follow are intended to illustrate and not to limitthe inventive concepts presented herein.

SPECIFIC EXAMPLES Example 1

SiC fibers were coated with AlN by a chemical vapor deposition (CVD)process. The CVD coating process involved the use of AlCl₃ and NH₃ asprecursors. Solid AlCl₃ was heated to about 120-150° C. and the AlCl₃vapor that was generated was transported by hydrogen gas flow to the hotzone where the NH₃ was introduced. The AlCl₃ react with NH₃ to produceAlN. The typical deposition temperature was about 850-1000° C. Thepressure in the deposition chamber was about 50 torr. The resulting AlNcoating thickness varied from 5-15 microns. FIGS. 1 and 2 show these AlNcoated SiC fibers.

A green body of AlN coated SiC fiber reinforced composite was fabricatedby a slurry filtration process. The slurry was prepared by ball millingSiC powder (Submicron Arendal) in water. The solids content of theslurry was about 75 wt %. The pH of the slurry was adjusted to 9 byadding ammonium hydroxide. About 0.5 wt % of sodium silicate was used asa temporary binder. Mixing was accomplished by ball milling for 24hours. About 4 grams of slurry was poured into a glass mold 0.25×0.25×1inch which was placed over filter paper in a Buchner funnel.

About 1.75 grams of AlN coated SiC fibers in the form of a bundle wasdipped into a diluted SiC/water slurry with 50 wt % SiC and 0.1 wt %Triton X-100 surfactant to surface treat the fibers and facilitateinfiltration. The treated bundle was then placed into the glass moldcontaining the undiluted slurry. A vacuum was drawn on the funnelcontaining the mold until the cast was fairly dry.

The cast was allowed to fully dry. The cast was then placed in a furnaceand completely densified by conventional reactionbonding/siliconization. Complete densification was obtained at 1500° C.in 1 hour under vacuum.

The volume percent of coated fibers for this sample was estimated to beabout 30%. The sample was fractured in a standard four point bend test.Fiber pull-out was observed as shown in FIG. 5.

Example 2

SiC fibers were coated with BN by a CVD process. The average coatingthickness was about 2 microns. The composite was fabricatedsubstantially in accordance with Example 1. The volume percent of coatedfibers was about 12%.

The composite sample was fractured in a standard four point bend test.Fiber pull-out was observed as shown in FIG. 6.

To test the crack deflection of the composite, a notch was cut in acomposite sample having the dimensions 1/8"×1/4"×2". The sample was thensubjected to a four point bend test. Stable crack growth was observed asshown in FIG. 7. In FIG. 8, the load-deflection characteristics for a BNcoated SiC fiber reinforced reaction bonded SiC composite were comparedto those of a monolithic reaction bonded SiC composite. The SiC fiberreinforced composite shows stable crack growth as evidenced by thenon-linear composite behavior. The monolithic reaction bonded SiCcomposite, on the other hand, shows catastrophic failure at a load ofaround 33 lbs. The stable crack growth of the SiC fiber reinforced SiCreaction bonded matrix is attributed to fiber pull-out.

Comparative Example 3

A composite was fabricated substantially in accordance with Example 1,except that uncoated SiC fibers were incorporated into the reactionbonded SiC composite. No fiber pull-out was observed as shown in FIG. 9.

The foregoing examples are not intended to limit the subject invention,the breadth of which is defined by the specification and the claimsappended hereto, but are presented rather to aid those skilled in theart to clearly understand the invention defined herein.

What we claim is:
 1. A method for manufacturing a fiber reinforcedceramic composite, comprising the steps of:a) providing a preformcomprising ceramic fiber comprising silicon carbide and having porosity,and b) infiltrating particles consisting essentially of ceramicparticles into the porosity to form an infiltrated body consistingessentially of:i) a preform comprising ceramic fiber comprising siliconcarbide and having porosity, and ii) ceramic particles infiltrated intothe porosity of the preform.
 2. The method of claim 1 wherein theinfiltrated ceramic particles are silicon carbide.
 3. The method ofclaim 2 wherein the ceramic fiber has a non-oxide coating thereon. 4.The method of claim 3 wherein the non-oxide ceramic coating comprises acompound selected from the group consisting of aluminum nitride, boronnitride, and titanium diboride.
 5. The method of claim 3 wherein thenon-oxide ceramic coating comprises boron nitride.
 6. The method ofclaim 5 wherein the boron nitride has a thickness of between 0.5 um and2 um.
 7. The method of claim 2 wherein the ceramic fiber comprisingsilicon carbide has thermal stability at a temperature of at least 1420°C.
 8. The method of claim 2 wherein the fiber comprising silicon carbideconsists essentially of silicon carbide and has thermal stability at atemperature of at least 1420° C.
 9. The method of claim 2 furthercomprising the step of:c) substantially completely densifying theinfiltrated body to form a composite comprising:i) ceramic fibercomprising silicon carbide, and ii) a matrix surrounding the ceramicfiber, the matrix consisting essentially of a continuous phase and aninfiltrated SiC ceramic particulate phase.
 10. The method of claim 9wherein the step of densifying includes siliconizing the infiltratedbody to form a SiC fiber reinforced composite having a substantiallycompletely dense matrix consisting essentially of silicon carbide andsilicon.
 11. The method of claim 10 wherein the preform is infiltratedwith a mixture comprising silicon carbide powder and water prior to thestep of siliconizing.
 12. The method of claim 11 wherein the mixturefurther comprises a surfactant.
 13. The method of claim 9 wherein thestep of infiltrating includes the step of infiltrating a slurrycomprising silicon carbide powder and water, and wherein the water isremoved from the infiltrated body prior to step c).